1. Field of the Invention
The present invention relates to a positively-charged electrophotographic organic photoreceptor, and more particularly, to a multi-layered positively-charged organic photoreceptor having improved sensitivity and electrostatic properties.
2. Description of the Related Art
Double-layered positively-charged electrophotographic organic photoreceptors basically include an electrically conductive support, a charge transport layer, and a charge generating layer.
Since the charge generating layer is thin, it may be easily worn due to friction between the charge generating layer and toner, and friction between the charge generating layer and a cleaning blade. To overcome this problem, an overcoat layer may be further formed on the charge generating layer. Also, an adhesive layer for improving adhesiveness between the electrically conductive support and the charge transport layer, or a charge blocking layer for preventing migration of charges, may be further formed between the electrically conductive support and the charge transport layer.
An electrophotographic imaging process using such a double-layered positively-charged organic photoreceptor is as follows.
After positively charging the surface of an organic photoreceptor, a laser beam is used to irradiate a charge generating layer to generate positive and negative charges. Positive charges (holes) are injected into a charge transport layer by applied electric field to the organic photoreceptor and migrate to an electrically conductive support. The negative charges (electrons) migrate to the surface of the organic photoreceptor to neutralize the surface charges. A surface potential varies by exposure, so that a latent image is formed in an exposed region. Then, this latent image is developed with toner, thus forming an image on the surface of the photoreceptor. The thus-formed image is transferred on the surface of a receptor, such as paper or a transfer belt.
Compared with single-layered organic photoreceptors requiring complex electrical properties for a single layer, double-layered positively-charged organic photoreceptors including two layers responsible for different functions may more easily control electrical properties, such as charge potential and exposure potential.
Also, since a stable electric field may be applied to the thin surface layers of double-layered positively-charged organic photoreceptor, the photoreceptor may retain a larger amount of charge at a given field strength and develop images with a larger amount of toner. Thus, such a double-layered positively-charged organic photoreceptor may be advantageously used for development of a liquid toner having a small particle size and a large charge amount.
However, when a composition to form the charge generating layer of the double-layered positively-charged organic photoreceptor is coated on the charge transport layer, the charge transport layer is partially dissolved in an organic solvent contained in the composition, resulting in a change in the thickness of the charge transport layer or elution of components of the charge transport layer. The change in the thickness of the charge transport layer reduces the charge potential of the organic photoreceptor.
To resolve these problems, use of a solvent incapable of dissolving components of a charge transport layer as an organic solvent of the composition for forming a charge generating layer has been suggested.
However, the thus manufactured double-layered organic photoreceptor has a definitely defined interface between a charge transport layer and a charge generating layer, so that charges generated at the charge generating layer by a laser beam may not be injected into the charge transport layer. Thus, a surface potential of an exposed portion is not sufficiently reduced. In addition, an exposure potential may continuously increase with repeated use.
Japanese Patent Laid-open publication No. hei 2-97961 discloses a negatively-charged electrophotographic photoreceptor having a charge generating layer and a charge transport layer sequentially stacked on an electrically conductive support, wherein the charge generating layer comprises a charge generating pigment having a hole transporting function and a fluorene compound in a binder resin.
However, the conventional organic photoreceptor is still open to improvement because it is not satisfactory in view of the sensitivity with increased printing speed and the exposure potential and the residual potential with repeated electrophotographic processes.